If you are trying the USB-stick method, the stick has to be formatted as FAT32, but I have found that recent images are actually too big to be flashed by USB stick anyway. I couldn't get it to work on my (similar) Gigablue UHD UE 4K. I googled around and found that it was a size issue.
There is a 'rescue' mode you can use instead, where you can flash it from the browser of another device on your network, such as a Windows PC. This works fine with recent images.
To use rescue mode, make sure the box has a wired (not wi-fi) network connection, then switch on the power at the back whilst pressing the front-panel menu button. The LCD should say "enter setup?" and a number will count down. Keep pressing the menu button the whole time until it says it's entering setup. Then it will give an IP address to enter into your browser on your PC. On your PC you can then flash the firmware over your network.
NB: I didn't know about this rescue method when I needed to re-flash recently, after a boot-loop issue, so I ended up using an old image from years ago that was still on my PC! I put that on a USB stick, and it flashed OK because it was small. Then I was able to flash the latest one via the menus.